Radiation imaging apparatuses such as X-ray imaging apparatuses have been used widely for diagnoses of medical conditions in medical sites. In particular, the radiation imaging apparatuses for an intensifying screen—X-ray film wherein high sensitivity and high image quality have been attained in a long history are still used in the medical sites all over the world.
In recent years, a digital type radiation image detector represented by a flat panel radiation detector (FPD) is also making an appearance, which makes it possible to obtain radiation images as digital information, to conduct image processing freely, or to transmit image information instantly.
The radiation image detector has a so-called “scintillator panel” that converts radiation into fluorescence. The scintillator panel is one that receives radiation transmitted through an object and emits instantly fluorescence having intensity corresponding to a dose of the radiation, and it has a structure in which a phosphor layer is formed on a substrate.
FIG. 2 shows a cross-sectional configuration diagram of a flat panel detector (FPD) possessing conventional scintillator panel 6. The scintillator panel is fitted with radiotransparent substrate 1, metal reflective film 2 formed on one of the substrate surfaces, protective film 3 covering the metal reflective film, an alkali halide phosphor layer formed on the protective film provided on the metal reflective film as a number of acicular crystals via evaporation (CsI:Tl in which Tl is doped as emission center), and external protective films 5A and 5B as moisture resistance layers covering the phosphor layer.
As shown in FIG. 2, scintillator panel 6 is closely brought into contact with photoelectric transducer array 7 and pressed with buffer material 8 on which front plane 9 for electromagnetic shielding is placed to subsequently seal the inside of PDF with housing 10.
X-ray passing through the affected area of a patient enters from the front plane 9 side of FPD, and is converted into light with phosphor 4. Light thereof is read by photoelectric transducer array 7 to obtain an image, but the same amount of light as light going the photoelectric transducer array side is also emitted on the front plane side, since light from phosphor 4 is isotropically emitted. Metal reflective film 2 serves to reflect light on the photoelectric transducer array side so as not to waste this emission.
Incidentally, protective film 3 covers one of the surfaces of the metal reflective film in FIG. 2. On the other hand, provided is a metal reflective film as well as a structure possessing a protective film covering up to at least the side wall of a substrate (refer to Patent Document 1, for example).
FIG. 3 shows a cross-sectional schematic diagram of a scintillator panel described in Patent Document 1. Reflective film 2 and substrate 1 are enclosed and covered by protective films 3 and 5.
The reason is as follows. The scintillator panel inside FPD is possibly exposed for a few days at high temperature and high humidity (a temperature of 60° C. and a humidity of 70%, for example). It is assumed in this case that a halogen element produces I3− ion via water penetration in such a way that CsI+I2=Cs++I3− since the phosphor contains the halogen element, and Al is positively ionized via reaction with the metal reflective film conventionally made of Al to be dissolved out of the metal reflective film. Or, possibly, it is also assumed that a hydroxide ion is generated via reaction with a H2O molecule by ionizing I in such a way that H2O+I−→HI+OH−, and at this time, Al is dissolved in the following reaction such that Al+3OH−=Al(OH)3.
There is a problem such that intensity unevenness is generated to an image since light reflection intensity is lowered at the portion where corrosion is generated.
Further, light generated at the portion other than the scintillator inside a flat panel detector strays into the inside of the scintillator (stray light), whereby noise is possibly made, or fluorescence is possibly generated by impurities other than Tl. Since light in this case becomes noise with respect to a diagnostic image, inherent light emission (CsI:Tl) of the scintillator exhibits a wavelength of 400-700 nm as shown in FIG. 4, and it is considered that noise can be suppressed if light other than the foregoing light is designed not to reflect at the reflective film.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2003-262671.